This invention relates generally to marine geophysical prospecting of earthen substrata beneath bodies of water and, more particularly, to a marine geophysical prospecting system for sensing energy converted by selected substrata.
Marine seismic exploration is usually conducted by towing a seismic streamer at a given depth through the ocean or other body of water. The streamer is provided with a plurality of pressure sensors, such as hydrophones, disposed at appropriate intervals along the length thereof. Acoustic wave energy is provided in the vicinity of the cable by an air gun or other suitable source means. Acoustic energy from the source travels through the water and downwardly through the earth with a portion of it being reflected upwardly at strata interfaces where there is a contrast in the acoustic impedance characteristics between two strata. The plurality of reflections (or refractions) of the source waves generates a sequence of upwardly traveling waves that are distributed in time. The pressure sensors in the streamer cable detect the pressure waves produced in the water by the upwardly traveling waves and provide electrical signals indicative thereof to suitable processing and recording equipment located on a seismic vessel that is towing the streamer cable.
Conventional seismic techniques have difficulty in resolving closely-spaced substrata. Additionally, such conventional seismic methods, while providing high-quality information on the geological structure of the subsurface, do not easily provide information about porosity, permeability, and fluid saturation. Consequently, more accurate information on these parameters is continuously sought by explorationists. It has been found that the conversion at subsurface interfaces of acoustic energy to electrical energy by the streaming potential or of electrical energy to acoustic energy by the "electroosmotic effect" yields additional information on these parameters not easily obtained from conventional seismic techniques. The present assignee's related application, Ser. No. 07/696,059, and U.S. Pat. No. 4,904,942, detail electroseismic systems which overcome many of these disadvantages by employing electroacoustic and acoustoelectric techniques, respectively, to gather data on land. However, these systems do not address the use of the electroacoustic or acoustoelectric technique in the offshore environment, the characteristics of which differ greatly from those of a land-based setting.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and an improved method is provided for marine geophysical prospecting of substrata beneath bodies of water.